1. Field of the Invention
The present invention generally relates to electronics and, more specifically, to radio transceiver systems. The present invention more specifically relates to a multiband coupler.
2. Discussion of the Related Art
A coupler is generally used to recover part of the power present on a so-called main or primary transmission line for another so-called coupled or secondary line, located nearby. Couplers are divided in two categories according to whether they are formed of discrete passive components (it is then spoken of lumped element couplers) or of conductive lines close to one another to be coupled (it is then spoken of distributed couplers). The present invention relates to the second category of couplers. The ports of the main line are generally designated as IN (input) and OUT (output). Those of the coupled line are generally designated as CPLD (coupled) and ISO (isolated).
In many applications, part of the power transmitted over a line needs to be used, for example, to control the power of an amplifier in a transmit system, to control the linearity of a transmit amplifier according to the losses linked to the reflection of an antenna, to dynamically match an antenna, etc.
The main parameters of a coupler are:
the insertion loss, which represents the transmission loss between ports IN and OUT of the main line (the insertion loss is then defined with the two other ports CPLD and ISO of the coupler loaded with a 50-Ω impedance);
the coupling, which corresponds to the transmission loss between ports IN and CPLD (the coupling is then defined with the two other ports OUT and ISO loaded with a 50-ohm impedance);
the isolation, which corresponds to the transmission loss between portions IN and ISO (the isolation is defined with the two other ports OUT and CPLD loaded with a 50-ohm impedance);
the directivity, which corresponds to the difference in transmission loss between ports ISO and CPLD, from port IN; and
the matching, which represents the reflective loss on the four ports.
An ideal coupler has an infinite directivity, that is, no power is present on the port of its secondary line located in front of the output port of its main line when a signal flows from the input port to the output port of this main line. In practice, a coupler is called directional when its directivity is sufficient (typically greater than 20 dBm) for the powers recovered on the accesses of its secondary line to enable to distinguish the power flow direction in the main line. When the two ports of the secondary line of the coupler are used to simultaneously have the power information, the coupler is called bidirectional.
Radio transceiver devices are more and more capable of operating in several frequency bands. Such is for example the case in mobile telephony, where cell phones have evolved from dual-band to tri-band, and now to quad-band.
The transceiver chain then comprises as many paths as the device is capable of processing frequency bands at once in transmit and receive mode. Each path is associated with a coupler sized according to the frequency band to be processed. In particular, the lengths of the main and secondary lines depend on this frequency band. This need for a different sizing of the couplers complicates the manufacturing. Further, with couplers of different length, the directivity varies from one coupler to the other, which is not desirable.
In a coupler, if the two ports of its secondary line and the output port of its main line are perfectly matched, no parasitic reflection occurs. Such a perfect matching can unfortunately not be obtained in practice. In particular, the port from which the portion of the power is sampled by coupling is seldom ideally matched. As a result, parasitic reflections generate errors on the recovered data.
A mismatch of the port of the secondary line of the coupler from which the information is sampled may have different origins. Most often, the coupler is placed on an insulating substrate (for example, of printed circuit type) to be associated with other circuits. It is then not possible to guarantee a perfect matching (typically 50Ω) of the measurement port (CPLD). Further, if the couplers have different sizes, this matching risks to vary from one coupler to another.
Further, in a multiband coupler, the antennas connected at the output of the main lines introduce an additional coupling. The greater this coupling (the poorer the isolation between the two antennas), the more altered the measurement results. The coupler is then not sufficiently frequency-selective for one path over the other.